Of course it's a shame that Laurent needs to eat and of course this has everything to do about RM being able to build for Mac or not. Knowing him, I know he would prefer RM to be open source but it would be much harder for him to monetize his efforts.
I'm perfectly ok with paying a fee to get RM but I know that wouldn't pay for support (especially if the community handles it which is already the case).
Corporate sponsorship doesn't always works and often lacks security and stability for both the developer and the project as they are at the mercy of one entity.
There is still a lot that you can do to contribute back to the project. Much of the build system and various supporting libraries have been open-sourced. Furthermore, it is the stated goal of RubyMotion that development of higher-level abstraction libraries should be driven by the community.
I suspect that the number of people who would actually be interested in (or, indeed, even capable of) contributing back to the parts of RubyMotion that are closed source is rather small.
Probably true. But as an amateur developer, I'd like to try it out... yet I'm not in a spot where I can just drop $200 to see if I like something or not.
Besides the obvious difference of target platforms, the biggest difference you'll run into IMO is that MacRuby is a full Ruby 1.9 implementation on top of Objective-C, whereas RubyMotion is more of a dialect of Ruby. i.e. eval doesn't exist in RubyMotion, you can't require gems at runtime, can't use MiniTest or RSpec, etc.
I think this is a shame, because I suspect lots of people would quite like to contribute back to the project, and that's currently not an option.